Basic corrosion in boilers results primarily from the reaction of oxygen with the metal. Stresses, pH conditions and chemical corrosion have an important influence and produce different forms of attack.

Where is corrosion usually experienced?

Corrosion may occur in the feedwater system as a result of low pH water and the presence of dissolved oxygen and carbon dioxide. On-line boiler corrosion occurs when boiler water alkalinity is too low or too high. When oxygen-bearing water contacts metal, often during idle periods, corrosion can occur. High temperatures and stresses in the boiler metal tend to accelerate the corrosive mechanisms. In the steam and condensate system, corrosion is generally the result of contamination with carbon dioxide and oxygen. Additional contaminants such as ammonia or sulfur-bearing gases may increase attack on copper alloys in the system

What measures are taken to prevent boiler system corrosion?

The most common methods for prevention of corrosion include:

• Removing dissolved oxygen from the feedwater,

• Maintaining alkaline conditions in the boiler water,

• Keeping internal surfaces clean,

• Protecting boilers during out-of-service intervals, (Cortec’s Critical Care Line)

• Counteracting corrosive gases in steam and condensate systems with chemical treatment (Cortec’s S-10)

Internal Treatment

Chemical treatment of water inside the boiler is essential whether or not the water has been pretreated. Internal treatment, therefore, compliments external treatment by taking care of any impurities entering the boiler with the feedwater (hardness, oxygen, silica, iron) regardless of whether the quantity is large or small.

In some cases external treatment of the water supply is not necessary and the water can be treated by internal methods alone. Internal treatment can constitute the sole treatment when boilers operate at low pressure, much of the condensate is returned and the raw water is of good quality (Cortec’s VCI 616 & VCI 617). However, in moderate and high-pressure boilers, external pretreatment of the make-up water is mandatory for good results. With today's higher heat transfer rates, even a small deposit can cause tube failures or wasted fuel.

The purpose of an internal treatment program is fivefold:

• To react with incoming feedwater hardness and prevent it from precipitating on the boiler metal as scale.

• To condition any suspended matter such as hardness sludge in the boiler and make it non-adherent to the boiler metal.

• To control the causes of boiler water carryover

• To eliminate oxygen from the feedwater.

• To provide enough alkalinity to prevent boiler corrosion.

In addition, a complete treatment program should prevent corrosion and scaling of the feedwater system and protect against corrosion in the steam-condensate systems.